Many types of hand held power tools are known which have a rotatingly driven spindle via which a tool or bit is rotatingly driven. One example of such a power tool is a rotary hammer having a hollow spindle within which an air cushion hammering mechanism is at least partly located.
Rotary hammers are known which have a housing and a hollow cylindrical spindle rotatably mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are generally provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Rotary hammers are known which can be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. Such rotary hammers generally have a hammer only mode in which the spindle is locked against rotation.
Some rotary hammers include an overload clutch in the drive train for rotatingly driving the spindle. One design of overload clutch comprises a drive gear which extends co-axially around the spindle or around a mode change sleeve rotatingly fixed on the spindle. The drive gear can be rotatingly driven by an intermediate shaft, which intermediate shaft is driven by the motor of the hammer. The drive gear rotatingly drives the spindle via a set of ratchet teeth, which are engageable with opposing ratchet teeth formed on the spindle or alternatively on the mode change sleeve. The drive gear is axially biased to engage the opposing sets of ratchet teeth by a spring. An axial end stop is required, either around the spindle or around the mode change sleeve, against which the spring bears in order to axially bias the drive gear. The end stop is generally provided by a washer, which washer is maintained in its axial position by a circlip which extends around an annular recess formed in the external surface of the spindle or the mode change sleeve.
Where such a mode change sleeve is used, the mode change sleeve is generally axially slideably mounted on the spindle. A mode change mechanism acts on the mode change sleeve to move it axially, to move the drive gear mounted on the mode change sleeve into and out of engagement with an intermediate shaft depending on whether rotary drive to the spindle is required. To simplify the mode change mechanism, a spring may be used, to bias the mode change sleeve into its position in which the drive gear engages the intermediate shaft, then the mode change mechanism need only move the mode change sleeve axially in one direction, to move the drive gear out of engagement with the intermediate shaft with the spring acting to return the mode change sleeve to its position in which the drive gear and the intermediate shaft are engaged. Again, an axial end stop is required around the spindle against which the spring bears in order to axially bias the mode change sleeve. The end stop is generally provided by a washer, which washer is maintained in its axial position by a circlip which extends around an annular recess formed in the external surface of the spindle.
The problem with circlips is that they are difficult to correctly assemble onto the hammer spindle or mode change sleeve. If the circlip is not correctly assembled then the axial stop is not effective and the hammer will not operate correctly. Also, if the circlip is not correctly assembled it is likely to come loose and this is likely to cause damage to the hammer when it is first used.
Similar overload clutches to those discussed above can be used on other types of hand held power tools, such as drilling machines, where the clutch extends around the spindle. Again the use of circlips in such overload clutches can give rise to the problems discussed above.